Key Technology of UHV Transmission and Its Research Status

Abstract: The construction of UHV power transmission project can meet the demand for rapid power growth in China in the future, and meet the network connection requirements for the gradual expansion of power grid scale and the formation of cross-regional networking. This article describes the research status of UHV transmission technology at home and abroad. Based on the actual situation in China, it analyzes the overvoltage and insulation coordination and external insulation characteristics of the Chinese 1000kV UHV AC transmission project and ±800kV UHV DC transmission project. Electromagnetic environment and UHV equipment manufacturing and inspection, testing, etc. face or urgent problems to be solved. According to the analysis, relying on the first-class UHV AC test base and UHV DC test base of the State Grid Corporation of China, the above problems can be well studied and solved, and the test data and analysis results can be directly used to guide the design of UHV transmission projects in China. Construction.

Introduction China has a vast territory, and the distribution of energy reserves and power loads is extremely uneven. In power grid construction, energy distribution and power transmission are always issues that need to be considered comprehensively. With the economic development, the establishment of long-distance, large-capacity, low-loss transmission systems has become an inevitable development of China's power grid [1]. In addition to the large-scale and long-distance transmission of electric energy, the UHV AC and DC transmission grids can also greatly improve the safety, reliability, flexibility, and economy of the grid, with significant social and economic benefits. However, the higher the operating voltage level of the grid, the higher the corresponding technical requirements [2]. The ±800kV UHV DC demonstration project in Fulong to Shanghai Nanhui, Sichuan, and the 1000kV UHV AC pilot demonstration project in Jindongnan-Nanyang-Jingmen were successively approved by the National Development and Reform Commission for construction of UHV AC/DC transmission technology in China. Basic experimental facilities need to be improved. The construction and gradual investment of the UHV AC test base and DC test base of the State Grid Corporation of China will enable China to have a reliable test and test platform, and at the same time be able to provide UHV AC and DC transmission technologies in China for engineering design and construction. Significant support. This paper studies some key issues of UHV transmission technology in view of China's national conditions.

1. Status Quo of UHV Transmission Technology Research 1.1 Current Situation of UHV Transmission in Foreign Countries The United States, Japan, the former Soviet Union, Italy, and Brazil began the study of UHV AC and DC transmission technologies in the late 1960s and early 1970s. Corresponding laboratory and short-distance test circuit were built [3]. In the area of ​​UHV AC transmission, the Soviet Union began construction of a 1150 kV transmission project connecting the Siberian, Kazakhstan and Ural grids in the 1980s, becoming the first (and only) country in the world with proven experience in UHV transmission operations. . In the 1990s, due to changes in its domestic economic situation and power demand, the project was stepped down to 500kV operation. Japan, Italy, and other countries have also launched UHV AC transmission projects. However, due to slowing world economic development and changes in the international environment, the electricity demand in these countries has stagnated or even declined. UHV power transmission projects have been discontinued and high pressures have been built. Transmission lines can only operate at low voltage levels. In terms of UHVDC transmission, the highest operating voltage level in the world today is the Itaipu ±600kV DC transmission project in Brazil, and this voltage level still has a large gap with the ±800kV class DC transmission project constructed in China.

1.2 The research status of China's UHV transmission technology adopts UHV transmission, which is of great significance for achieving a more optimal allocation of resources, improving the utilization rate of transmission corridors, and protecting the environment [1]. The air pollution in China is serious, and the sending of “Southwest hydropower” brings problems such as high altitude and ice cover. Therefore, the influence of climate and geographical environment on the electromagnetic environment indicators and external insulation characteristics is relatively large compared with the experimental environment and experimental conditions of foreign research institutions. Differences, foreign research conclusions are difficult to apply. More importantly, the 800kV UHV DC transmission project constructed in China is the world’s highest voltage level, largest transmission capacity, and longest transmission distance. The realization of this project must be based on the actual situation in China.

The UHV transmission technology research in China began in 1986. Over the past 20 years, China’s scientific research institutions have successively carried out the “distance transmission method and voltage level demonstration” and “preliminary demonstration of UHV transmission in UHV AC and DC transmission fields. "And the feasibility of using AC megavolt high voltage power transmission" and other studies, overvoltage levels in UHV transmission systems, insulation coordination, the impact of transmission lines on the environment and the choice of equipment, lines, towers, typical substations (conversion station) With regard to argumentation, preliminary results have been achieved.

With the development of China's power grid construction, the number of transmission lines is increasing, and the transmission line corridor will become increasingly tense, which will inevitably bring about the same arrangement of multiple AC/DC transmission lines, multiple conductors of DC transmission lines, and multiple DC transmission lines. Set up and other new issues. At the same time, the national environmental protection agencies are increasingly strict on the management of the electromagnetic environment. The design of UHV projects must meet the requirements of relevant management systems, laws and regulations. The construction of UHV transmission projects requires several stages such as UHV test and research, development of UHV equipment, and evaluation of UHV equipments, and construction of UHV AC test bases and DC test bases is the basis for UHV transmission technology research. . In order to meet the research needs of UHV transmission projects, in August 2006, the UHV DC test base of the State Grid Corporation of China was based on the Changping Park East District of Beijing Zhongguancun Science and Technology Park. The base is fully functional, efficient, and practical. The construction of the base closely integrates with China. Actual 800kV UHVDC transmission project. In October 2006, the UHV AC test base of the State Grid Corporation of China was based on the west side of Wuhan 500kV Phoenix Mountain Substation. The test line section of the test site includes single and double loops (currently live electrified operation). The tower layout is resistant to −straight−straight−resistant. The test line is used for the selection of conductors and ground lines. − The South-Jing test demonstration project remains consistent. After completion, the State Grid Corporation’s UHV test base will provide strong technical support for the construction and operation of UHV AC pilot demonstration projects and DC demonstration projects in China.

2. Research project of UHV test base 2.1 Overvoltage and Insulation Coordination During the operation of the UHV transmission system, it will withstand overvoltages caused by operational impact, fault impact, and lightning impact. As there is no UHV overvoltage standard in China, it is very important for the safe operation of the system to study the overvoltage and insulation coordination and choose the correct and economical way to reduce the overvoltage and insulation levels of the equipment.

Due to the special nature of the UHV transmission project, there are various options for the layout of the conductors. There are more insulator strings and tower head gaps than the EHV lines. Like a rod frame, the conductors are arranged horizontally and vertically, and the insulators I and V are arranged. Even Y string and so on.

The fact that China's UHV transmission lines cross the high altitude region also determines that air gap discharge voltage characteristics at different altitudes must be studied. Therefore, on the basis of conventional research projects, studying the discharge characteristics of air gaps under different conditions is of far-reaching significance in guiding the design of UHV transmission projects.

2.2 Contamination External Insulation Problems Usually, the main contents of the research on filthy external insulation are the artificial contamination of insulators and casings, rain, ice and external insulation tests under conditions of low atmospheric pressure (high altitude). The key to the test is at full voltage. Proceed. In the preliminary study of China’s UHV transmission project, due to the lack of high-voltage pollution power supplies, the tests of long string insulators for 1000 kV AC and ±800 kV DC lines had to be conducted abroad, such as Japan NGK and Sweden STRI.

There are many studies on UHV AC transmission in foreign countries, and there are 1150kV operating experience in the former Soviet Union for reference. However, taking into account that China's UHV transmission project passes through the geographical environment and geographical and climatic conditions (such as high altitude, heavy pollution, etc.), foreign construction experience should not be copied. Taking the 8-split conductor design adopted for China's current 1000kV AC transmission line as an example, according to the experience of foreign UHV operation, the UHV line needs at least 300kN large-diameter large-tonnage insulators. Because of the large differences in the pollution of insulators, the resistance to flashover, aging, and mechanical properties, the following four issues are the key issues affecting the design and selection of external insulation for UHV power transmission projects:
(1) The pollution flashover resistance of the external insulation of equipment. Unlike foreign UHV power transmission projects, China has a vast territory and complex geographical environment. There is a big difference in the pollution situation in different regions. At the same time, the level of air pollution in our country is much more serious than that of the United States, the former Soviet Union and other countries. With the rapid development of China's economy and the increasingly serious industrial pollution, it is foreseeable that in the future 1000kV AC UHV and ±800kV UHV power transmission projects, the problem of pollution discharge of external insulation of the line will become more and more serious. Therefore, UHV When the transmission line is designed for external insulation, the first thing that should be considered is the resistance to pollution flashover of the external insulation of the equipment.
(2) The performance and reliability of composite external insulation. In the face of increasingly serious air pollution problems, it is unavoidable that a large number of composite insulators are used in UHV AC/DC transmission projects. However, research on UHV transmission projects in foreign countries is relatively weak in this regard and there are not many lessons to be learned. Therefore, in-depth and meticulous research is needed on the problems of composite insulators such as electromechanical properties, aging, mechanical fatigue, and internal defect detection, as well as the external insulators of composite insulators under high altitude and icing conditions. In addition, when some circuit insulators and substations use RTV coatings (including PRTV coatings) to increase the pollution flashover resistance of the equipment's external insulation, the definition and criteria of the reliability, service life, and RTV coatings (including PRTV coatings) also need to be further improved. the study.
(3) Outer insulation string length and string type selection. For transmission lines, the string length of the insulator is the basis for determining the tower head size and tower head structure design. Good insulators with good flashover resistance have a relatively small string length. Conversely, insulators with poor contamination resistance and flashover performance will result in long strings of insulators, and the project cost of transmission lines will inevitably increase greatly. Considering the experience of domestic and overseas UHV transmission line projects and the analysis of their operating characteristics, China's UHV transmission lines may use different insulator arrangements, such as V-shaped strings, than ordinary transmission lines. Since the insulator contamination and flashover voltage of the V-shaped string are different from those of the suspended string, it is necessary to solve the problem of the selection of the outer insulation level under different strings.
(4) High altitude and icing problems. Compared with the plain areas, the problem of pollution and insulation in high altitude areas is even more serious. The developed countries in the world are mainly in the plain areas and rarely encounter high altitude problems. Therefore, they have done very little research in this field. Only countries such as the former Soviet Union, Japan, Canada, and Sweden have done some high-altitude external insulation pollution. Flash characteristics short string or model test. Because China's UHV AC and DC transmission lines will often pass through high-altitude, ice-covered and other climatically degraded areas (such as the planned Xiangjiaba-Suonan ±800kV UHVDC transmission line, the maximum altitude of the line segment reaches 3680m). Therefore, under various environmental conditions, the reasonable insulation configuration of transmission lines is also a key issue in the study of UHV transmission technologies. The key to this issue is still to be carried out at full voltage level.

2.3 Electromagnetic Environment Study The electromagnetic environment issue is a major technical issue that must be considered in the design, construction, and operation of UHV AC and DC transmission lines. At this stage, this issue is still a hot topic in the world [3]. The electromagnetic environment of an UHV transmission line is related to the corona characteristics of the transmission line. The corona of the line can cause corona loss, power frequency electromagnetic fields (AC transmission), DC electric field effects (DC transmission), radio interference, and audible noise. as a result of. Among them, the audible noise problem has become the most important factor in the design of UHV transmission lines. At high altitudes, the electromagnetic environment of transmission lines is more serious than that of plain areas. However, the research on this issue at home and abroad is not sufficient. Therefore, how to effectively use the pollution and pressure vessel laboratory of the UHV AC test base and the DC test base to develop the electromagnetic environment of high-altitude transmission lines is also a new topic.

Compared with the ±500kV DC transmission project, the UHV DC transmission project has the characteristics of high voltage, large conductor cross section, high iron tower, and wide single-circuit corridor, so its electromagnetic environment has certain differences from the ±500kV DC line. Since the electromagnetic environment of the UHVDC transmission project is closely related to the type of conductors and the height of the overhead lines, the specific choices will not only directly affect the project's technical plan and construction cost, but also directly affect the environmental protection of the project construction.

At present, ±800kV UHVDC power transmission has no precedent in the world, there is no experience to learn from, and the corresponding electromagnetic environment has no relevant standards to follow. On the electromagnetic environment of the 1000kV AC UHV transmission line, it mainly involves power frequency electric field, power frequency magnetic field, radio interference and audible noise, although some research results have been applied to the Jin-nan-Jing test. Demonstration project, but measures to reduce corona loss, reduce audible noise, and radio interference still need further study.

In order to carry out UHV DC electromagnetic environment research, the State Grid Corporation of China built a UHV DC test line section with a length of 900m and a double-circuit test voltage ±1200kV adjustable line layout, and a set of dimensions 70mx22mx13m. Two-compartment corona cages (both monopolar and bipolar) can be used. A UHV AC test line with a length of about 1 km (one for each single circuit and one for dual circuits) is built in Wuhan. 35m x 8m x 8m corona cage.

2.4 The status quo of equipment 2.4.1 Development of China's power transmission equipment manufacturing industry After 50 years of development in China's power transmission equipment manufacturing industry, high-voltage, ultra-high voltage and UHV power transmission and transformation equipment from scratch, from small to large. With the continuous improvement of China's power grid technology and equipment, the technological level and localization capabilities of related equipment manufacturing companies have also been continuously improved. In recent years, under the guidance of the goal of “manufactured by localization of equipment manufacturing”, after the practices of ±500kV DC transmission project, 750kV AC transmission project, and Three Gorges project, China’s power equipment manufacturing industry is in product design, manufacturing, and testing and testing technologies. With the rapid development, basically meet the needs of China's power construction.

At present, domestic equipment manufacturers have the ability to develop and supply UHV power transmission and transformation equipment such as conductors, fittings, lightning arresters, capacitive voltage transformers and capacitors, and suspension insulators; and UHV transformers, reactors, and high-speed earthing switches. And switching equipment still need to accelerate the development of research and development; UHV casing, GIS, controllable reactors and other equipment is difficult to manufacture, the country currently has no manufacturing capacity, in the early stages of the construction of UHV transmission projects, the need to introduce foreign products.

2.4.2 Long-term electrification assessment, operation status inspection and overhaul technology The decision-making of UHV power grid is a strategic and directional decision, which is very challenging [1]. Long-term electrification assessment of equipment ensures the system The safe and reliable operation is of great significance [2]. Considering the high altitude, heavy pollution, and bad weather conditions in China's power grid regions, this puts higher requirements on the insulation level and technical conditions of UHV equipment. Whether it is domestic R&D or direct introduction, it needs to be long-term. Charged assessment.

The level and capability of the detection and maintenance technology of UHV equipment are the reliability indicators for the operation of UHV transmission and transformation projects. The detection of operational status, especially the application of on-line monitoring technology, can prevent problems. However, due to the fact that our experience in related areas still has major deficiencies, we evaluate the existing on-line monitoring technology, select a reliable operation state detection and maintenance technology, and propose corresponding maintenance items, testing cycles, inspection tools, and inspection standards. And specifications and so on.

3. Some Key Technologies Research Results and Conclusions 3.1 Overvoltage and Insulation Coordination 3.1.1 Air Gap Operation Impact and Lightning Impact ± 800 kV DC Transmission Tower Air Clearance Operational Impact and Lightning Impact 50% Breakdown Voltage Tests in China Power Science The outdoor test site of the institute (6000kV/300kJ impact voltage generator) and the National Grid Corporation UHV DC test base outdoor test field (7200kV/480kJ impact voltage generator). Changing the length of the cross tie between the strings of insulators and the length of each side of the V-shaped string can give different air-to-gauge distances from the conductor to the cross arm and the conductor to the column.

3.1.2 System over-voltage level UHV AC power system power frequency over-voltage level: 1.3 circuit breaker substation side 1.3pu. Among them, 1.0pu=11002/3; 2 line circuit breaker side 1.4pu, the duration is less than 0.4s. UHV AC transmission system operation over-voltage level: 1 substation: 1.6pu of ground, phase-to-phase circuit breaker inside/outside is 2.8/2.9pu; 2 circuit: 1.7pu for ground, and 2.9pu for phase-to- phase.

To test long-term operational reliability, short-duration power frequency withstand voltages were introduced. The short-duration power frequency withstand voltage and pressure mode of UHV AC transformers and GIS equipment in China are as follows: Transformer: 1.5 pu × 1 h + 3 pu × 5 min + 1.5 pu × 1 h; GIS: 1.5 pu × 30 min + 3 pu × 1 min +1.5pu x 30min.

The insulation coordination of the UHV transmission system is based on the arrester operation and the level of lightning surge protection. Considering the coordination factor (safety margin), the operability of the electrical equipment is used to determine the operation of the electrical equipment and the lightning impulse withstand voltage. According to the calculation and analysis results, the lightning strike rate of UHV AC general transmission lines should not be less than 0.1 times/100kmâ‹…a, and the lightning safe operation of ultra-high voltage AC large-span lines should not be less than 50 years per year. The UHV AC substation (switching station) is intruded by lightning. Safe operation of waves should not be less than 1500~2000 years.

3.2 Pollution external insulation selection Through different kinds of ultra-high voltage 1000kV class AC insulators and ±800kV class tributary insulators under atmospheric pressure and different pressures, different types of pollution and different pressures were found. There is a large difference between the curve of insulator salt density and flashover voltage, and the relationship between altitude and flashover voltage. Combining the specific conditions of the UHV lines, in terms of the pollution flashover resistance of different types of insulators, the resistance to flashover is: composite, three-umbrella, double-umbrella, glass, rod-shaped porcelain, common, and the last four are not much different (AC test results).

The data obtained from the tests have been used for the selection of external insulation for equipments in lines, substations, and converter stations in China's 1000kV UHV AC transmission and ±800kV UHVDC transmission projects, including specific pollution areas, different altitude areas, and different coverage. Types of insulators, umbrellas, strings, number of strips, length of string, and minimum creepage distance of the ice area. It is necessary to pay special attention to the fact that in the UHV AC and DC transmission systems, it is necessary to carry out research on the external insulation of the full voltage level, and the pollution and environmental laboratory to be built in the UHV test base of the State Grid Corporation is providing Reliable technology research platform.

3.3 Electromagnetic Environment Study According to the preliminary research results of the ±800kV DC project in China, the ground composite field strength below the line should be no more than 30kV/m, and the ion current density should not exceed 100nA/m2. These two kinds of corona effects are greatly affected by the height of the conductors to the ground, so they may not be used as a basis for restricting the selection of conductors. Wire selection is mainly determined by the radio interference level and noise level it produces.

According to the provisions in [1], the 50% value of the audible noise generated by the corona should not exceed 45dB (A) at 20m away from the projection of the positive pole wire of the overhead overhead transmission line to the ground; when the altitude is greater than 1000m, the Not more than 50dB(A). The test results in the UHV DC test base show that, according to the line design and operation conditions, the ±800kV DC test line with 6 × 720mm2 conductors can meet the design and operation requirements even when the relative humidity is high, which is lower than the standard. 13.3%~17.7% (Night time), but it is also lower than the limit of 58dB(μV/m) in terms of audible noise, that is, the 6×720mm2 wire determined during the feasibility study phase of UHV engineering is satisfactory in terms of radio interference and noise. Electromagnetic environment needs.

It should be noted that the audible noise of the transmission line is related to the wire structure, the height of the wire to the ground, humidity, temperature, wind speed, season and other factors. The audible noise of the old and new wires will also be different. It is necessary to clearly grasp the overhead transmission of UHV. The range of audible noise generated by line corona requires long-term monitoring and statistics.

At present UHV DC test sites are undergoing several tests to synthesize field strengths and ion currents in order to study the distribution characteristics and variation laws of synthetic field strengths and ion currents under different weather, humidity, pressurization time, and ageing conditions.

3.4 Equipment manufacturing, inspection and operation testing Technical specifications and test standards for high-voltage transmission equipment currently being developed, supervision guidelines for electrical equipment, on-site handover testing methods and standards, on-site preventive testing methods and procedures, and on-site testing equipment technical conditions Such standards and protocols can directly guide the design, production, acceptance, and inspection of equipment.

With the successive scheduling of UHV AC and DC equipment, the long-term electrification assessment and inspection and overhaul technologies will be necessary for the UHV equipment before and during its operation. The construction of the electrified assessment test circuit of the UHV AC test base and the DC test base of the State Grid Corporation of China is meeting various needs for electrified assessment.

4. Conclusions In order to build a national UHV power transmission project, a large number of UHV test studies and UHV equipment assessment and test work are required. The establishment of the UHV AC test base and DC test base of the State Grid Corporation of China is an UHV AC pilot demonstration project in China. The DC and DC demonstration projects provide a first-rate test and test platform.

At present, China has made certain progress in overvoltage and insulation coordination, external insulation characteristics, electromagnetic environment, and long-term electrification assessment, inspection, and operation detection technologies of UHV power transmission projects in UHV power transmission projects. Some of the tests and data analysis have been obtained. The results can be directly used for the construction of UHV AC and DC transmission projects in China.


[1] Liu Zhenya. UHV power grid [M]. Beijing: China Economic Publishing House, 2005.

[2] Liu Zhenya. Research record of UHVDC transmission technology [M]. Beijing: China Electric Power Press, 2006.

[3] Liu Zhenya. Album of research results on UHV AC transmission technology[M]. Beijing: China Electric Power Press, 2006.

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